For years, astronomers have thought that the first stars born in the universe formed alone, but new simulation results suggest that some of them may have had partners.
The first stars began as cold clumps of hydrogen and helium gas that started to coalesce into stellar embryos. Many of them were huge, astronomers think. Telescopes can't yet peer back far enough in time to see this first generation of stars, but computer simulations can model their formation and tell astronomers what they might have looked like.
These simulations indicated that these earliest stars would have been whoppers, with masses hundreds of times that of our sun, and would likely have formed in single-star systems.
"While binaries had been proposed, they'd never been seen in cosmological simulations," said Matthew Turk, a graduate student at Stanford University who ran the new simulations.
Binary and even triple star systems are common in the more modern universe, and they are typically much less massive than early-generation stars.
In the new simulations, Turk and his colleagues — Tom Abel, also of Stanford, and Brian O'Shea of Michigan State University — included more precise information to better mimic the conditions thought to have existed in the early universe.
When they ran the simulation, they got what Turk called a "rather surprising result": They saw indications of fragmentation in the gas clouds that suggested the formation of a binary star system.
"When we started the simulation, we didn't know it was going to form a binary," Turk told Space.com.
Just how many early stars might have been binaries isn't clear yet — determining their abundance will take many more simulations.
"The idea here is that it's not forbidden, that it seems like [the stars] form alone, but sometimes they might have some friends, they might form in pairs," Turk said.
The finding, detailed in Friday's issue of the journal Science, also jibes with observations of very old, low mass stars in the halo of the Milky Way, O'Shea said.
These early stars and galaxies later merged to form today's galaxies, such as our own Milky Way. By observing them and analyzing their compositions, astronomers can estimate the average masses of the earliest stars, O'Shea explained.
The estimates from this method suggested that the first stars weren't quite as massive as expected, with the average falling around 30 to 40 solar masses.
The new simulations explain the discrepancy between the massive stars seen in simulations and the estimates of much smaller masses: "Instead of one big star, you get two smaller stars," O'Shea told Space.com.
The finding that early stars might have been binaries could have implications for the detection of gamma ray bursts in the early universe.
Gamma rays and gravity waves
These energetic explosions are thought to occur when a massive star rotates very fast and then collapses. The doomed stars are spun up by companion stars that strip off their partner's outer layers.
Having binaries in the early universe suggests that gamma ray bursts also occurred early on. When NASA's James Webb Telescope is launched into space in 2013, it will be able to peer further back into the universe's past than any other telescope, and should be able to see the supernovas that go along with the gamma ray bursts, O'Shea said.
The existence of early binaries could also have implications for the detection of elusive gravity waves, disturbances predicted by general relativity.
Gravity waves from single black holes are difficult to detect above normal noise — to pick up the signal — assuming any exist — astronomers need a dynamic, evolving system, which is just what a pair of black holes would provide.
The team plans to do more simulations to get a better picture of just how common binary star systems might have been in the early universe and what the likely masses of these stars might have been.